Im not a big camera guy and i am still in the process of learning so i doubt i would be able to tell the difference.
From the link above
The Hero 3 was equipped with an A7, whereas the Hero 4 will supposedly have an A9 chip. The GoPro Hero 3 had 4K recording features at the rate of 15 frames per second. It is rumored that as the GoPro Hero 3 makes use of the A7 Hybrid DV/DSC, 1080p60 Camera SoC, it is but natural that the new camera will be upgraded to the A9. This will mean that the GoPro will be able to capture 4K videos, at the rate of 30 frames per second. Currently, there is no other camcorder that can capture 4K videos, at this rate. In addition, the A9 will have the capability of capturing footage of 1080p at the rate of 120 frames per second and footage of 720p at the rate of 240 frames per second. This kind of recording will result in a stunningly smooth slow motion recording, due to the high frame rates. In fact, it is the ideal portable camera to have, for taking recordings under water and in other extreme sports and other such situations.